Cultured mouse fibroblasts which are transformed by RNA viruses, a DNA virus or a chemical agent, all secrete a 35,000 Mr-phosphoglycoprotein (major excreted protein, MEP) in large amounts. Nontransformed murine fibroblasts secrete this protein in much lower amounts but can be stimulated to synthesize and secrete MEP by treatment with tumor promoters or growth factors. We have purified MEP, prepared monospecific antisera against it and cloned a cDNA which codes for MEP. The protein contains mannose 6-phosphate, the lysosomal recognition marker. It is processed intracellularly to give two specific lower molecular weight forms, the lowest of which has a predominantly lysosomal localization. Transformation and tumor promoters stimulate MEP synthesis by increasing levels of MEP specific mRNA. We are studying this system as a model of regulation of lysosomal protein synthesis, processing and secretion as it is affected by transformation and agents which mimic the transformed state, such as tumor promoters.